This fixes a crash with the following steps to reproduce:
1. highlight a vertex or shape with the cursor
2. swap image using keyboard controls (i.e. press D)
3. left click on image without moving cursor. No shape may be below
cursor.
What happens is that the canvas will emit a selection signal for
the shape from the previous image, which then can't be found anymore
in `shapes_to_items`, causing this error:
```
Traceback (most recent call last):
File "/home/stefan/code/labelImg/./labelImg.py", line 806, in shape_selection_changed
self.shapes_to_items[shape].setSelected(True)
KeyError: <libs.shape.Shape object at 0x7fdb281d06a0>
```
This builds upon the fix from #858, which fixed the issue for
selected shapes, but not highlights.
The use case is adjusting shapes by moving their vertexes, even if
they are adjacent or overlapping another shape. Currently always
the "later drawn" shape wins, making it impossible to adjust an
earlier shape without re-creating it. Adjusting existing shapes is
common for review scenarios.
I have experimented with preferring the highlighted shape, but
I found it finicky and less clear as to how the vertex preference
works. JOSM (and OpenStreetMap editor) also uses the "prefer
selected" approach and it works well there.
For any shape with a dimension smaller than 2*epsilon, there can be
multiple potential points to select from. In practice this resulted
in e.g. the top right corner being highlighted when the cursor was
placed below the bottom right corner.
Steps to reproduce this particular issue:
1. draw a box
2. select the box
3. hover over one of the corners as if you want to resize the box (no click needed)
4. press DEL to delete the box
5. left click on the canvas
Note there are two more destructive operations on the canvas' `shape` in
`undo_last_line` and `reset_all_lines`, but former is dead code and latter
I don't know how to trigger. Thus I can't be sure un-highlighting will not
lead to visual glitches.
There are other potential fixes, e.g. checking in `selected_vertex`. I can
rework the patch, but then again, the fix is so straight forward that telling
me is probably more effort than just rewriting it.
See #750
See #605
Stefan Breunig